Metastatic dormancy is defined as the ability of rests of metastatic cancer tissue to survive, but not invade and progress at distant sites. In thyroid cancer, patients with small lung metastases often survive for decades without radiographic or clinical progression, suggesting that restraint of progression at metastatic sites is intrinsic to many thyroid cancers. Because most thyroid cancer- related deaths are due to late-stage progressive metastatic disease, it is crucial to define mechanisms that by which these cancers escape from metastatic dormancy. Metastasis suppressors are negative regulators of cancer metastasis and growth that may serve a gate-keeping role in metastatic progression. By interrogating the KiSS-1/GPR54 metastasis inhibitory signaling cascade in cancer cells, we identified a motility-suppressor role for regulator of calcineurin 1-4 (RCAN1-4) in vitro; and demonstrated loss of this protein in metastatic thyroid cancer tissue samples. RCAN1-4 has also been shown to play a central role in VEGF-induced endothelial cell growth and motility. Interestingly, the RCAN1 (DSCR1) gene that encodes all RCAN1 isoforms is located on chromosome 21, and is one of many genes overexpressed in Down's syndrome (trisomy 21). It was recently demonstrated that Rcan1 plays a critical functional role in the reduced solid tumor incidence and progression associated with Down's syndrome. In mouse two Rcan1 isoforms are expressed that are homologous to the two dominant primary human isoforms, RCAN1-1 and RCAN1-4. Recent in vivo studies confirmed that short form of Rcan1 in mouse, which is homologous to human RCAN1-4, is the primary induced isoform in the neovasculature of tumor grafts, suggesting that it may be specifically important in tumor angiogenesis induced by VEGF However the functional role of this isoform on cancer progression has not been directly tested in vivo. The overall hypothesis of this proposal is that RCAN1-4 is a key gate-keeper that restrains thyroid cancer progression by inhibiting cancer cell invasion and by inhibiting endothelial cell response to VEGF and other angiogenic signals. We will use a variety of in vivo models to test this hypothesis.